1. Field of the Invention
The present disclosure relates to battery charging/discharge circuits and battery pack protection, and more particularly, to battery charging/discharge circuits capable of trickle precharge and/or trickle discharge. Utility for the present invention can be found in battery charging/discharging/protection systems for portable electronic devices, for example, laptop computers, PDAs, cell phones, and/or any type of electronic device having a rechargeable battery.
2. Description of the Related Art
Rechargeable batteries, especially lithium ion batteries, need to precharge (recovery-charge) from deeply discharged status to avoid stressing the depleted batteries. When a rechargeable battery is deeply discharged and its cell voltage becomes lower than a threshold voltage VUV, it cannot be directly charged using large charging current. Instead, a precharge mode is needed. In the precharge mode, a small charging current is used, until the battery voltage is charged larger than the voltage VUV, and then it can be charged in normal mode, i.e. charging by larger charging current. For lithium ion battery, the threshold voltage VUV is approximately 2.4V˜3.0V for one cell, depending on battery type and manufacturer. The precharging current is about 10 mA˜100 mA. However, the normal charging current can be a few hundred milli-Amperes to 1 Ampere depending on the battery capacity.
FIG. 1A depicts the charging profile 50 for a lithium ion rechargeable battery. When the battery voltage is higher than VUV, the battery enters into constant current (CC) charging mode, and a large constant current is used to charge the battery (the battery voltage also increases as the battery capacity increases). When the battery voltage increases to VOV, which represents overvoltage (normally around 4.2V for a lithium ion battery), the battery enters into a constant voltage (CV) charging mode. In this mode, the charger holds the voltage at VOV. When the charging current decreases to a predetermined minimum value, for example 50 mA, the charge procedure is stopped. During the CV charge mode, the charger must regulate the voltage precisely to VOV (to within +/−0.005 V), otherwise the charging current will not taper off with increasing battery capacity. If, for example, the charging output is larger than VOV, then over-charging the battery may occur, which may present safety issues with lithium ion batteries.
The conventional circuit 10 to implement precharging is shown in FIG. 1B. A precharge MOSFET 12 in series with a resistor 14 is used for precharging. At the time of precharging, charge FET 16 turns off and precharge FET 12 turns on. Therefore, the precharging current is approximately determined by the voltage difference between charger input voltage VPACK+ and total cell voltage Vcell, VPACK+−Vcell, divided by the serial resistor 14 Rpre. When an AC adapter (not shown) is present and VPACK+ is higher than the cell voltage Vcell, the charging or precharging will start based on the initial voltage of each cell. If the voltage in any cell is lower than the threshold VUV, the battery pack will be in the precharging mode. Otherwise normal charging will be taken.
Those skilled in the art will recognize that the circuit 10 of FIG. 1B includes a battery monitor IC 20 that includes circuitry to monitor voltage and current conditions on each of the cells (Cell1, Cell2 . . . Cell4) of the battery pack 22. Such circuitry may include a switching network 24 to sample each cell voltage. To control the operation of the precharge MOSFET 12, the conventional circuit 10 includes a comparator 26 that compares a constant reference voltage 28 (VUV) with the voltage across each cell, via switches 30.
However, one drawback of the topology depicted in FIG. 1B is that an extra power MOSFET (i.e., MOSFET 12) and resistor 14 are required, which are expensive and increase PCB area. Additionally, with this topology, the lower the cell voltage results in a larger precharging current. Also, the precharging current decreases with the increasing of cell voltage, which translates into longer time to finish precharging.
Additionally, the value of the resistor 14 is typically fixed, and the maximum and minimum precharging current is also typically fixed, and cannot be adjusted to accommodate different battery pack requirements.
Another drawback of this topology is that the battery pack 22 and the MOSFETs are vulnerable to an abnormal condition, such as the VPACK+ terminal is shorted to the VPACK− terminal, or an external reversed charger is attached to the VPACK+ and VPACK− terminals. With this topology, a discharge FET 18 is either turned on to allow discharge or turned off to disable discharge. When the discharge FET 18 is turned on, if an abnormal condition occurs, a large current may be drawn from the battery pack 22 to flow through the discharge FET 18 and the charge FET 16, which in turn will damage the battery pack 22 and/or the MOSFETs.
Alternatively, when the battery pack 22 is removed from an electronic system, for example, and put on a shelf, the discharge FET 18 may be turned off to protect the battery pack 22 from the abnormal condition. However, since the discharge FET 18 is turned off, the battery pack 22 will fail to power the electronic system immediately when the battery pack 22 is plugged back into the electronic system, and hence a mechanical method or an electronic circuit may be needed to inform the circuit 10 to turn on the discharge FET 18. The additional mechanical method or electronic circuit will increase the complexity, price and/or size of the circuit 10. Additionally, the battery pack is still vulnerable to damage caused by the abnormal condition after the battery pack is plugged into the electronic system.
A conventional solution for the battery pack protection is that the discharge FET 18 is turned off to avoid the large current when the abnormal condition happens. After being turned off for a predefined period, i.e. 30 seconds, the discharge FET 18 is turned on again. If the abnormal condition still exists when the discharge FET 18 is turned back on, the large current will flow through the discharge FET 18 and trigger the battery pack protection again. Consequently, the discharge FET 18 is turned off again. Otherwise, the battery pack 22 will operate in a normal discharge mode with the discharge FET 18 turned on. However, if the abnormal condition exists for a long period, the large current will flow through the discharge FET 18 continuously, which will eventually damage the battery pack 22 and/or the MOSFETs.
Thus, it is desirous to have a circuit and method thereof that is capable of trickle precharge and/or trickle discharge, and it is to such a circuit and method that the present invention is primarily directed.